1. Field of the Invention
The present invention is invention relating to an optical transmission and receiving equipment. More particularly, the present invention relates to optical transmission and receiving equipment and a control method thereof, using a semiconductor laser or the like as the optical source, and having an interference filter, for removing so-called background optical noise, interposed between the optical source and optical receiving instrument
2. Description of the Related Art
Before now, optical transmission and receiving equipments using semiconductor lasers were controlled as noted below. Specifically, optical transmission and receiving equipments required a prescribed optical filter for eliminating background optical noise during light transmission and reception. The wavelength of light emitted from the semiconductor laser varies depending on the temperature of the semiconductor laser itself. Consequently, the optical filter needs to be an interference filter with a bandpass broad enough to allow for the variation in laser beam wavelength. However, while optical filters with broad bandpass do transmit laser beams with a broad range of wavelengths, these filters do not eliminate background optical noise effectively. Meanwhile, when using a narrowband bandpass filter, the temperature of the semiconductor laser may be controlled to improve the effective removal of background optical noise, whereby the variation in wavelength of the laser beam, corresponding to the temperature variation, is reduced. However, this requires the addition of means for controlling temperature to the optical transmission and receiving equipments. As a result, the equipments become large and complex and those costs become high.
A prescribed laser radar equipment 100 is disclosed in Japanese Patent Laid-open Publication No. 62-103587. In this publication, it is proposed that this laser radar equipment 100 does not use special means for controlling temperature or the like, but does use a narrowband bandpass interference filter 104, which is very effective in removing background optical noise. As a result, the equipment automatically controls the tilt angle of the aforementioned interference filter 104, so that the transmission of the laser beam transmitted by the interference filter 104 continues to be maximized.
FIG. 7 is a block diagram showing the constitution of the aforementioned laser radar equipment 100. The figure shows an example of the laser radar equipment 100 which varies the tilt angle of the interference filter 104. Moreover, the laser radar equipment 100 is provided an interference filter 104. In the laser beam transmitted by this interference filter 104, the central wavelength (most easily transmitted wavelength), providing the maximum transmission, varies according to the incidence angle of the laser beam. The central wavelength tends to become shorter when the incidence angle of the laser beam in relation to the interference filter 104 increases. The prior art is constituted on the basis of this principle. This is explained here in more detail. As shown in FIG. 7, the laser beam 111 emitted from a semiconductor laser 101 passes through a beam splitter 102 and strikes a target object (not shown). Part of the aforementioned laser beam 111 is reflected by the beam splitter 102 and branched off as a reference beam 112. This branched reference beam 112 is then reflected by a reflecting mirror 103 and becomes the reference laser beam 113. This reference laser beam 113 passes through the interference filter 104 and strikes a detector 105. At this time, the angle of the interference filter 104 in relation to the angled laser beam can be adjusted. Also, a reference information signal detected by the detector 105 is input to a discriminator 106.
The discriminator 106 determines the transmittance of the aforementioned reference laser beam 113 by the interference filter 104 on the basis of the input reference information signal. Then, this discriminator 106 generates a prescribed control signal (angle signal). The control signal is for controlling the tilt angle of the interference filter 104; the control signal is calculated so that the transmittance of the laser beam by the interference filter 104 remains maximized. This control signal is input to a tilt angle controlling instrument 107. The tilt angle controlling instrument 107 rotates the interference filter 104 according to the control signal thus provided. The tilt angle is thereby adjusted so that the transmittance of the laser beam by the interference filter 104 becomes the maximum transmittance.
Meanwhile, the laser beam 114 reflected from the target object passes through the narrowband bandpass interference filter 104, along an optical path parallel to the light path of the reference laser beam 113, and reaches a optical receiving instrument 108. In this way, the reflected laser beam 114 reaches and passes through the interference filter 104 at an incident angle, such that the transmittance by the interference filter 104 is maximized.
In this laser radar equipment 100, temperature change of the semiconductor laser 101 results in a change in the wavelength of the laser beam emitted. When the wavelength of the laser beam deviates from the central wavelength of the interference filter 104, the tilt angle of the interference filter 104 is automatically adjusted by the discriminator 106 and the tilt angle controlling instrument 107. The central wavelength of the interference filter 104 is thereby controlled that the oscillation wavelength of the semiconductor laser 101 falls within the bandpass range of the interference filter 104. Consequently, even though the wavelength of the laser beam varies according to the temperature of the semiconductor laser 101, the maximally transmitted, reflected laser beam 114 reaches the optical receiving instrument 108.
Also, an optical information signal reproduction device is disclosed in Japanese Patent Laid-open Publication No. 61-61449. This optical information signal reproduction device is provided an interference filter, inclined in relation to the optical axis in such a manner that the light transmittance is halfway between the maximum and minimum values, in the light path of the beam reflected from the signal surface in a recording medium. The focus of a converging lens is controlled using a signal attained on the basis of the change in transmittance occurring in the zone between both sides of the optical axis by the divergence and convergence of the light flux reaching the interference filter. At the same time, the optical information signal reproduction device uses a heating and cooling element, which is thermally connected to the semiconductor laser, and controls the wavelength of light from the semiconductor laser, using a signal corresponding to the change in the intensity of transmitted light occurring on the entire surface of the interference filter according to the wavelength variation of the semiconductor laser.
Also, an example of an automatic birefringence meter is disclosed in Japanese Patent Laid-open Publication No. 61-210920. This automatic birefringence meter comprises a polarized light/transmitted light measurement portion which passes light, having little polarization and being output from a laser or monochromatic optical source comprising an interference filter or optical source and spectrometer, through a rotary polarization element to an arbitrary portion of a panel of birefringent material, then passes the transmitted light through a rotation analyzer and measures that light with a photodetector.
However, in the conventional optical transmission and receiving equipment shown in FIG. 7, the angle of the interference filter 104 is adjusted on the basis of a reference beam 112 drawn from a laser beam 111. For this reason, the semiconductor laser 101, interference filter 104, optical receiving instrument 108, and so forth must be arranged together in the same area. Consequently, this conventional equipment cannot be applied to a equipment wherein the semiconductor laser on the transmitting side and the optical receiving instrument on the receiving side are placed in distant locations, such as in an optical transmission equipment, or to a similar optical transmission system.
Also, in the conventional optical transmission and receiving equipment shown in FIG. 7, part of the laser beam 111 is removed as the reference beam 112, then reflected, and then transmitted by the interference filter 104 as the reference laser beam 113. At the same time, the laser beam 114 reflected from the target object travels along a path parallel to the transmission path of the reference laser beam 113, and is transmitted by the interference filter 104. For this reason, the position of each element and the optical axis and so forth must be established with great precision. The degree of freedom in the constitution of the device is strictly limited and the equipment is therefore difficult to build.